Conventionally, bearings for bearing main shafts of machine tools rotatably are requested to have properties of high rigidity, high rotational accuracy and low heat generation in order to keep the machining accuracy.
From the point of view of high rigidity, cylindrical roller bearings are generally used as bearings of machine tools. In recent years, single row and double row cylindrical roller bearings withstanding high speed rotation and having long lives have been desired from a rapidly increasing demand for improvement in productivity.
A so-called machined cage made from a copper alloy is used as a cage of a cylindrical roller bearing in the background art. However, when the machined cage is in use under high speed rotation, there is a disadvantage that the inner and outer circumferential surfaces and the pocket inner surface of the cage touch the rolling contact surface of an inner ring or an outer ring and cylindrical rollers so as to be worn to generate abrasive metallic particle. When the abrasive metallic particle is mixed into grease, there is a problem that the lubricating performance is degraded conspicuously to thereby cause seizing or damage.
In recent years, a cage made from synthetic resin has come to be adopted as a cage for solving the foregoing problem. The cage made from synthetic resin is formed by injection molding out of a raw material having a proper amount of reinforcements such as glass fiber or the like mixed into synthetic resin such as polyamide resin or the like.
The cage made from synthetic resin is indeed superior in abrasion resistance, but inferior in rigidity and strength to a cage made from metal. When the cage made from synthetic resin is used in a bearing rotating at a high speed, there is a possibility that the cage is damaged by a force acting on the cage.
Therefore, Japanese Patent Laid-Open No. 166544/1999 has proposed a cage 1 made from synthetic resin, in which a large number of cantilever-like columnar portions 1b are provided at predetermined intervals to project laterally from an annular portion 1a molded like a ring, as shown in FIGS. 31 and 32.
The same publication has disclosed a double row cylindrical roller bearing 5 using the cages 1 made from synthetic resin. That is, cylindrical rollers 2 each retained rotatably in a pocket portion between adjacent ones of the columnar portions 1b are disposed to roll desirably between an outer ring 3 and an inner ring 4.
In each synthetic resin cage 1 of the double row cylindrical roller bearing 5, an excessive circumferential force acting on the columnar portion 1b from the cylindrical roller 2 in accordance with the high speed rotation is absorbed by the elastic deformation of the columnar portion 1b by use of the elasticity of the synthetic resin. Thus, the cage 1 is prevented from being damaged.
According to the aforementioned cages 1 in the background art, when the bearing 5 rotates at a high speed, a large centrifugal force proportional to the square of the rotational speed acts on the cages 1 so that the cantilever-like columnar portions 1b are displaced in the arrow B direction (outer diameter direction) as shown in FIG. 33. With this displacement, the annular portions 1a are distorted so that the cages 1 are deformed elastically,
According to the inner ring guide type cages 1 shown in FIG. 33, due to the elastic deformation, interference occurs between the outer diameter sides of the outer side surfaces 1c of the annular portions 1a, between the outer side surface 1c of the inner diameter portion of each annular portion 1a and the inner ring 4, and between the inner diameter side of the inner side surface 1d of each annular portion 1a and the side surface of each cylindrical roller 2. Thus, there occurs a strong pressing force therebetween. This pressing force is proportional to the square of the rotational speed. Thus, with the higher speed rotation, the pressing force increases at an exponential rate so that a large force acts.
When rotation is made at a high speed in the state where these portions are pressed strongly, heat is generated due to friction so that the deterioration of charged grease or lubricating oil due to heat is accelerated. Thus, the lubricating performance is degraded. In addition, there is a problem that the contact portions of the cages 1, the inner ring 4 or the cylindrical rollers 2 are worn locally so that the life of the bearing is shortened.
In addition, also in an outer ring riding cage or a roller riding cage, interference portions are indeed difficult from those in the inner ring riding cage, but there is a serious problem of heating, wearing and so on due to similar interference caused by the elastic deformation of the cage due to centrifugal force.
In addition, the interference of each part also causes a fluctuation of torque. Thus, there is a possibility that the stabilization of the rotational accuracy is obstructed.
Therefore, the present invention was developed in consideration of the foregoing problems. It is an object of the invention to provide a roller bearing and a double row cylindrical roller bearing in which occurrence of interference among cages, an inner ring and rollers is prevented in spite of elastic deformation of the cages due to centrifugal force acting on the cages during high speed rotation, so that the bearing is low in heat generation, with stands high-speed rotation with a very small amount of lubricant, and has a long life.